Nico has totally suffered more. Remember, he's from the 40's, lost his sister, and discovers that he's gay, which is a secret that would be unbearable in his time. He traveled through Tartarus ALONE, and got trapped in a bronze jar and almost died there. Need I say any more?
I absolutely don't understand the people who voted for Percy. Ok, being a demigod is not easy but can you compare what happened to Percy (who by the way is one of the very few demigods not to have lost his mortal parent) to what Nico went through ???